The Joint Transform Correlator (JTC) is well know to be one of the more convenient devices for correlating two images in the sense that there is no need to fabricate holographic filters, such as matched filters or Phase Only Filters. The classical joint transform correlator requires a quadratic-processor in the Fourier plane. Prior work on nonlinear detection and processing of joint transform signals has shown the great usefulness of this approach; see B. Javidi, "Nonlinear joint power spectrum based optical correlation", Appl. Optics,28,, 2358-2367 (1989). All photorefractive correlators to date were operated primarily in the linear regime and did not implement the quadractic nonlinearity. In order to implement the required nonlinearity digital processing has been used via a computer and/or a camera, see the aforesaid Javidi publication. These serial operations have compromised many of the advantages sought in parallel optical processing. We show here that the nonlinearities necessary to realize a phase only JTC can be accomplished in parallel and real time through an optical photorefractive limiting quadractic processor. Nonlinear joint transform correlation becomes fully parallel, self-aligning and fully optical for the first time.
The phase only filter in particular is superior to other filters such as matched filters for detecting signals embedded in clutter noise. In addition the phase only filter allows high discrimination ability. These properties have generated interest in phase only nonlinear joint transform correlators. The invention presented here is believed to be the first implementation of photorefractive in the area of phase only joint transform correlation operation. The results at this stage are still preliminary but can be improved through using different crystal cuts which is the subject of future development of this new type of device. The device can be rapidly optically tuned by changing the intensities of the input light beams to provide different types of filters desired for varying additive noise and clutter environments. The broader classical matched filter correlation peak is more appropriate for environments having strong additive noise and no clutter, whereas the phase only filter is preferred for environments with clutter. In the saturation regime, the device functions as an inverse filter which is appropriate where their is no clutter or noise as in a controlled robotics environment.